1. Field of the Invention
The invention pertains to the field of uninterruptible power supplies (UPS) for use in providing continuous power to electrical devices in the event of a primary power system failure. More specifically, the UPS is adapted for use in systems having two primary sources of power in addition to the UPS.
2. Discussion of the Related Art
UPS devices are useful in preventing unwanted failure in electrical devices due to failure of an incoming power source. For example, a power utility company for a state or municipality may have a large industrial facility for converting energy from fossil fuels into electrical power. The resultant electricity travels on power transmission lines until the electricity arrives at an ultimate point of use. Frequently, the transmission of power to the ultimate end point of use is disrupted, e.g., by a rolling blackout or weather conditions that damage the power transmission lines.
Disruption of electrical power sources cannot be tolerated in situations where a continuous power source is necessary for the operation of critical electronic devices or where the disruption is likely to cause harm. For example, disruption of power to a telecommunications server for use in emergency medical situations might result in the loss of human life. Many businesses are unable to function without their computer systems in an operational state. Critical data may be irretrievably lost due to an abrupt power failure.
A variety of UPS devices are known in the art. Simple UPS devices typically act upon a sensed power loss from a single power supply and are capable of supplying power from a battery or capacitor storage device for a very limited time while a system shutdown is effected. For example, U.S. Pat. No. 5,701,244 describes a UPS that may be plugged into an alternating current (AC) receptacle for dedicated use on a personal computer. U.S. Pat. No. 6,178,515 describes a multi-processor device that automatically performs a normal shutdown of a general purpose operating system when a feed current from a main power supply is interrupted. UPS devices may be purchased from a variety of commercial sources, such as American Power Conversion of West Kingston, R.I.
The use of a dedicated UPS on a single power line does not necessarily prevent system shutdown. UPS devices are only intended to function for a limited or transient period during which the UPS discharges its stored power. For example, a UPS attached to a personal computer may diagnose a power failure condition, quick-switch to provide battery backup power, and notify the personal computer that it is necessary to commence an orderly shutdown procedure, in order to prevent the loss of critical data.
Accordingly, critical electronic systems may be provided with a plurality of power supply lines. For example, a citywide power grid may operate as a primary power source, and an auxiliary generator system may operate as a secondary power source. When there are at least two main power sources, a dedicated UPS may be deployed on each source, as is described in U.S. Pat. No. 6,191,500. This dual deployment is duplicative and thus, expensive. Failure of either power supply or either UPS can induce a shutdown of an attached electrical device when one of the UPS devices diagnoses a power failure condition, even though the other power supply or UPS may be intact and operational.
In still other alternative UPS deployments for dual power source situations, a single UPS may be coupled with more than one of the incoming power lines. The UPS may be operably configured for switching between the power sources based upon sensing of power disruption. For example, U.S. Pat. No. 5,920,129 discloses a UPS having a solid state transfer switch that is used to provide a constant source of direct current (DC) power by switching between primary and secondary sources upon sensing a voltage inversion caused by disruption of the primary source. U.S. Pat. No. 6,184,593 describes a UPS that is used to switch between a main power supply and an auxiliary generator system when the main power supply fails. U.S. Pat. No. 6,175,510 discloses a direct conversion UPS that may be used to supply power while a secondary supply comes on line. This UPS contains at least four bidirectional switches that are governed by a controller based upon sensed changes in power conditions.
A major problem with having a single UPS connected to two power sources is that the UPS is a single point of failure. Thus, failure of the UPS can cause the power disruption that is sought to be avoided.
It remains a problem to implement a single UPS to manage a dual power supply system in a manner that prevents the UPS from becoming a single point of failure.